1. Field of the Invention
The present invention relates to a radio communications system, a base station, a mobile station, a transmission control method, and a transmission control program; for example, the present invention is suitable for application to a radio communications system employing a space-division multiplexing system.
2. Description of the Related Art
Recent years have seen an increase in traffic having high-speed and high burst property such as packet communications. As a radio communications system accommodating traffic of such high burst property, there is available the TDMA/TDD system fixedly performing frequency allocation which often entails inconvenience.
Consequently, as a multiple access system for applying a radio communications system having traffic of high burst property, the CSMA (Carrier Sense Multiple Access) system may be considered. In this CSMA system, if a mobile station desires to carry out transmission, whether or not any other mobile station is in the process of performing the transmission is determined in advance by carrying out reception of a carrier wave (this operation is referred to as “carrier sense”); and if a received power is less than a predetermined value, it is considered that the other mobile station is not in the process of performing the transmission (this status is referred to as “idle”), hence, the transmission is carried out.
On the other hand, if the received power from the mobile station exceeds the predetermined value, it is determined that the other mobile station is in the process of performing the transmission (this status is called “busy”), so that the station starts its transmission after the transmission of the other mobile station is complete or after a lapse of random time.
By these operations, the CSMA system makes effective use of an allocated frequency for traffic of high burst property, thereby contributing to improving the frequency utilization efficiency.
Now, on the other hand, techniques of improving receiving property by controlling the directivity of an antenna with an adaptive array antenna to eliminate interference waves and other factors are under research.
Take a radio communications system based on the TDMA/TDD (Time Division Multiple Access/Time Division Duplex) system for instance. It is considered to employ the TDD characteristic of utilizing the same frequency for both transmission and reception so that the transmission may be made by applying the directivity of an array antenna which is obtained when the base station receives a signal from the mobile station. Installation of the array antenna in the base station makes it possible to gain effects of the adaptive array antenna in both transmission and reception even if a normal omnidirectional antenna is provided in the mobile station.
Further, the PDMA (Path Division Multiple Access) system which divides a space by controlling the directivity by means of the adaptive array antenna so as to use the same frequency simultaneously has been considered. For example, if the PDMA system is applied to a communications system of the TDMA/TDD method having a total of 8 slots with 4 slots for uplink (transmission from the mobile stations to the base station) and 4 slots for downlink (transmission from the base station to the mobile stations), communications between a plurality of mobile stations and the base station can be carried out within one slot, thus enabling the frequency utilization efficiency to be improved as a whole.
In the PDMA system, since space division is performed by employing the directivity of an array antenna, it must be noted that if the PDMA system has directions of the two mobile stations in close proximity when viewed from the base station, the same slot cannot be allocated to these two mobile stations.
Consequently, the radio communications system in accordance with the PDMA system is so designed that the base station preliminarily-detects the direction of each mobile station so as to allocate the same slot to those mobile stations having a certain degree of angular difference.
If the CSMA system is applied to the above-mentioned PDMA system, a further improvement of the frequency utilization efficiency can be possibly accomplished.
However, the PDMA system is based on the premise of application to the TDMA/TDD system, and not suited to traffic with high burst property such as packet communications suitable for the CSMA system.
Namely, in CSMA system, since the transmission from other mobile stations is inhibited while the transmission is being carried out by one mobile station, the base station cannot detect an azimuth of plural mobile stations at the same moment. Herein lies a problem that space division multiplexing by means of PDMA system is not applicable at least to the uplink. at least to the uplink.